I think it was in "Word on the Street" (assigned reading for one of my linguistics courses, best assigned reading ever) that talked about this. The book talked about the history of attempts to create a singular non-gender-specific pronoun in English (E, etc.), and then said that, well, basically, we have one. Grammarians simply haven't come to accept it, but informal English largely has, and it will most likely eventually be the standard for use in the singular, too. After all, it happened for "you," which used to be plural only (thou was singular).